Data transmission networks are composed of a number of units, generally called "data terminal equipment" (DTE), or simply terminals or stations. These terminals or stations can be composed of computers. The terminals communicate with one another through a transmission system. Token ring networks are among the data transmission networks in widest use at the present time.
Recent technological developments in the transmission systems area are leading to increasingly frequent use of fiber optics to transport data in a transmission network from one terminal to another. Thus, loop networks are formed whose transmission medium is composed of optical fibers. Such networks include the Fiber Distributed Data Interface (FDDI) network as defined by the international standardization committees such as Committee X3T9-5 of ANSI (American National Standard Institute). The standards defined in these ANSI documents are about to be adopted by the ISO, the International Standards Organization.
These standards define, for example, the maximum total length of the fiber in the network at 200 km, the maximum distance between the stations furthest apart in the network at 100 km, the maximum distance between stations (2 km) and the maximum number of physical connections, 1,000. They also define the speed and off-line "flow", the format of the frames which must not include more than 4500 octets or bytes (8-bits long) of information, the nature of the signals constituting the control characters placed at the beginning and end of the frame, and the length of each of the groups of signals depending on their nature, as well as the code in which the information is written and transmitted from one station to another.
One of the many advantages of using optical fibers as the medium for transmission of information on a network is the high level of throughput obtained which can be on the order of 100 Mbit/s.
In order for a terminal to be able to transmit information on a token ring network (also known as a loop network), the token access method to the loop is used. A synchronous transmission is established in the loop network with resynchronization taking place as the token passes each terminal. Thus all the terminals are monitor terminals.
When no terminal has any data to send, a token circulates in the loop. To send data, a terminal intercepts the first token which it receives then sends in its place, in the form of a data frame, all or part of the data which it has to send. It then sends another token into the loop.
Accidental loss of the token can result from transmission errors or reconfiguration of the loop. To reestablish the circulation of a token in case of loss or following the initial start-up of the network, a monitor terminal continuously monitors whether another terminal is transmitting, or whether a token is circulating in the loop. If neither event occurs for a certain period of time, the monitor terminal reestablishes the initial state of the loop by emitting a token followed by filler signals which can be distinguished from the token.
Accidental duplication of the token can also take place as a result of transmission errors. In this case, return to normal is also easily achieved. Since the frames transmitted by any given terminal have at the head of the frame the terminal address, a given terminal can recognize on reception the frames which it itself transmitted.
After having intercepted the token to transmit its data, a terminal resends a token immediately after sending its last frame. If after a certain period of time, which is one of the network parameters, this beginning of the frame has still not been received, the terminal returns to a transparent state without resending a token. Subsequently, the monitor terminal detects the loss of a circulating token and re-establishes the circulation of the token as outlined above. The frame is the elementary block of information, transmitted by any station or terminal, which travels on the transmission network. The frame includes a start of frame delimiter, an end of frame delimiter, frame control characters the address of the station for which the message is destined (Destination address), the address of the sending station, the length of the data (Source address), and finally the useful data itself. More simply, a frame is composed of useful data which is bracketed in time by signals located at the head of the frame and by signals placed at the end of the frame. The signals placed at the head and at the end of the frame which bracket the useful data are called control characters.
It is known that a computer is composed of a central processing unit, or CPU, and various peripherals which can be magnetic peripherals such as disk drives or other peripherals called input/output (I/O) devices which allow for exchanging data (Cathode Ray Tubes (CRTs), printers, etc.).
The CPU is composed of at least one processor and a main memory to which this processor is connected, and an I/O processor which monitors the exchange of data between the memory and the various peripherals. The set of functional component elements that make up the computer such as the central processor or the I/O processors, random access memory and read only memory, I/O controllers or peripheral controllers, are all accommodated on a group of boards whose dimensions are generally standardized. Each board is composed of a printed circuit on which a plurality of integrated circuits is mounted. These boards are generally connected to one parallel bus which provides communication between the different processors, transport of data between the different boards, and electrical power for them. Among the buses which are in most frequent use at the present time is the bus called MULTIBUS.RTM. II (registered trademark of the Intel Company).
As its name indicates, the architecture of a MULTIBUS.RTM. II is structured about a main bus of the parallel type and secondary buses. Only the main bus is standardized according to Institute of Electrical and Electronic Engineers. Inc. (IEEE) Standard 1296. This bus is called the parallel system bus or PSB. When a computer is used as a network station, it is connected to it by an interface composed of a special board called a host card or an I/O host module. According to IEEE Standard 1296, the host card is connected to the PSB bus by a coprocessor such as a VL 82c 389 type (made by Intel) which communicates in the message mode with the other functional component elements of the computer and which communicate with one another through the PSB. Data transfer to the network is managed by a microprocessor on the host card with the data to be transmitted on the network being routed through the internal communication bus of the microprocessor. It is clear however, that the data transmission modes on the PSB bus and the internal communication bus of the host card microprocessor on the one hand, and on the FDDI network on the other are completely different in terms of the data transmission rate, the transmission protocols used, the write codes, the format of the control characters, and the format of the information itself. For example, transmission of information occurs in parallel on the internal communication bus of the host card microprocessor and in series on the FDDI network.
It is therefore necessary to adapt the data transmission on the internal communication bus of the host card to the transmission conditions on the network.